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    <title>Document</title>
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    <script>
        var a = 1;
        function box() {
            // var a;--声明提升-函数内部把使用var定义的变量提升到函数内部的最前面
            console.log(a);//undefined
            var a = 2;
            console.log(a);//2
            a = 200;
        }
        box()
        console.log(a);//1
    </script>

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